This application claims the priority of German Application No. 198 01 395.7, filed Jan. 16, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for the automatic charging pressure control using an exhaust gas turbocharger with a variably adjustable turbine cross-section and for the automatic exhaust gas recirculation control in an internal-combustion engine, particularly a diesel engine. The engine has an air flow sensor connected in front of the compressor of the exhaust gas turbocharger in the suction pipe. An exhaust gas recirculation pipe connects the exhaust gas pipe with the suction pipe. An automatic control device, in a first low load-rotational speed range, automatically controls the exhaust gas recirculation and, in a second higher load-rotational speed range, automatically controls the charging pressure in the suction pipe by a corresponding variation of the turbine cross-section.
In the case of an automatic control system of this type known from European Patent Document EP-A-0 747 585, the charging pressure is controlled in the first lower range by way of a characteristic diagram and, in the second higher range, this charging pressure is automatically controlled by means of the characteristic diagram values as the desired values. Independently thereof, in the first range, the exhaust gas recirculation is automatically controlled using an exhaust gas recirculation valve and is switched off in the second range.
Also in the case of other known systems, two control circuits for the exhaust gas recirculation and the charging pressure are only used independently of one another, in which case one control circuit is not controlled automatically, that is, it is operated only in a controlled manner when the other is operated.
Furthermore, particularly in the case of diesel engines, being able to implement sufficiently high exhaust gas recirculation rates by way of the available exhaust gas recirculation flow cross-sections presents a problem. For solving this problem, in a known manner, a throttle valve is used for the intake air throttling and for increasing the exhaust gas recirculation delivery gradient.
The known arrangement, in the case of which a control of the charging pressure as well as an automatic control of the charging pressure is provided, in addition to the automatic control of the exhaust gas recirculation which is independent thereof, results in a relatively high-expenditure and cost-intensive solution. Because of the independent automatic control of the charging pressure and the exhaust gas recirculation, there is the risk that the two systems operate against one another and thus lead to unsatisfactory results with respect to the exhaust gas and the performance.
It is therefore an object of the present invention to provide a system of the above-mentioned type which can be implemented in a simpler manner and at lower cost, and which, in all operating ranges, leads to reliable and optimal values with respect to the exhaust gas and the performance.
According to the invention, this object is achieved in that the automatic control system has a first controller which has a first controller output for the variation of the turbine cross-section, the command variable for the automatic exhaust gas recirculation control in the first low load-rotational speed range being the air flow rate in the intake pipe, and the command variable for the automatic charging pressure control in the second higher load-rotational speed range being the charging pressure in the intake pipe.
The invention advantageously utilizes the variable cross-section of the turbine of the exhaust gas turbocharger for generating the required exhaust gas recirculation delivery gradient while the exhaust gas recirculation flow cross-section is given. The automatic control of this variable flow cross-section therefore results in the setting of suitable exhaust gas recirculation rates. In the higher load and/or rotational speed range, the air flow rate command variable must only be replaced by the charging pressure command variable in order to switch over to an automatic charging pressure control. In this case, the recognition is utilized that, in the low load-rotational speed range, an automatic exhaust gas recirculation control which is as precise as possible and, in the upper load and/or rotational speed range, a charging pressure which is automatically controlled as precisely as possible are prerequisites for achieving the object of a pollutant emission which is as low as possible while simultaneously minimizing the fuel consumption.
By means of the measures described herein, advantageous further developments and improvements of the system according to the invention can be implemented.
When still in the second higher load-rotational speed range, in order to implement an automatic exhaust gas recirculation control in addition to the automatic control of the charging pressure, an exhaust gas recirculation valve is provided in the exhaust gas recirculation pipe which can be set by a second controller constructed as an exhaust gas recirculation controller. This second controller also has the air flow rate as its command variable, as in the case of the automatic control of the exhaust gas recirculation by the first controller in the first low load-rotational speed range.
Since, in principle, the second controller is not required in the first low load-rotational speed range, devices are expediently provided for switching off or deactivating the second controller during the transition from the second to the first range. During the switching-off or deactivating of the second controller, the exhaust gas recirculation valve preferably retains its actual control value-its actual control signal. This is true at least when a definable applicable value for the control value or the control signal has been exceeded.
When the first controller in the first load and/or rotational speed range reaches its minimal output, it can no longer control the exhaust gas recirculation. For this case, devices are advantageously provided for switching off or deactivating this first controller and simultaneously switching on or activating the second controller.
The air flow rate sensor is preferably constructed as a hot-film air flow rate sensor.
The turbine of the exhaust gas turbocharger arranged in the exhaust gas pipe preferably has a guide blade adjusting device for the variation of the turbine cross-section.
For sensing the charging pressure, a pressure sensor is connected behind the compressor of the exhaust gas turbocharger.
For dividing the operating range into the two load-rotational speed ranges, a characteristic curve is preferably provided which is a function of the rotational engine speed and of the engine load (or injection). This characteristic curve expediently has a hysteresis in order to prevent a back and forth switching between the ranges in the boundary area.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.